2nd Gen Non-Engine/Transmission Vacuum test

[Kalaehina]

Hey guys, since my truck has been down for the whole axle nut problem I've had time to address the vacuum problem I've also been having. So here's my question, I put a vacuum gauge on the vacuum line in the cab, passenger side after the canister valve thing, it reads 15, at the actual vale control near the driver side center, it reads off my gauge pass 30. I think it's supposed to be at 27. What am I doing wrong? Or am I doing it right and my readings are telling me the problem? I'm totally lost when it comes to this part of the truck. Thanks for any help... ... ... ... .